Glass-ceramic articles are traditionally prepared through the closely controlled heat treatment of precursor glass articles. On that account, glass-ceramic articles are customarily produced by following three general steps: first, a vitrifiable batch of a predetermined composition is melted; second, this molten mass is cooled to a temperature at least within the limits of, and normally below, the transformation range and simultaneously made into the form of a glass article having a desired configuration; and, third, this glass article is exposed to a predetermined heat treatment in order to cause the generation of crystals in situ. (The transformation range has been defined as the temperature at which a molten material is transformed into an amorphous mass; this temperature in general being estimated as being in the vicinity of the annealing point of a glass.)
During the crystallization heat treatment the glass is heated to temperatures above the transformation range of the glass and whichaapproach and ordinarily go beyond its softening point. It is well known that, in these conditions, the viscosity of the glass becomes sufficiently low so that the article becomes susceptible to thermal deformation. The severity of this phenomenon obviously increases as the temperature approaches the softening point of the glass and goes beyond it.
The crystals developed in a glass-ceramic exhibit a melting point higher than the softening point of the precursor glass. Consequently, by causing crystallization in situ during the heat treatment of a parent glass article in order to convert it into a glass-ceramic, care must be taken in raising the temperature above the transformation range of the glass to develop sufficient crystallization in order to provide an adequate internal structure to support the article, thereby minimizing thermal deformation. One must also realize that the composition of the glass remaining in the article continually changes as its components become an integral part of the crystals during the heat treatment process. In most cases the viscosity of the residual glass is greater than that of the parent glass. Nevertheless, thermal deformation is an ever present problem; particularly with articles having two dimensions which are large but have narrow cross sections, such as dinner plates. Such products have required the use of formers or supports during the heat treatment of the precursor glass article in order to assure the desired geometry in the final product.
Furthermore, the in situ crystallization of a glass article is effected more rapidly and to a greater extent as the temperature is raised above the transformation range and into the region of the softening point of the glass. Consequently, for reasons of industrial economy, it is necessary to raise the temperature of crystallization as rapidly as possible up to a temperature as high as possible. Such practice obviously increases the risk of thermal deformation of the precursor glass article. Therefore, continued research has been carried out to discover glass compositions capable of rapid in situ crystallization and exhibiting only minimum, and preferably practically no, thermal deformation. These researches have been particularly active in the area of tableware articles where a primary objective has been to remove the need for formers to support the articles during the heat treatment of the parent glass bodies.